The present invention relates to a method of recycling phosphate ester-based functional fluids. More particularly, this invention is directed to a process for recovering a trialkyl phosphate ester from an trialkyl and aryl phosphate ester-based functional fluid such that the recovered trialkyl phosphate ester is essentially free of aryl-containing phosphate esters and epoxide-containing additives present in the functional fluid mixture.
Organic phosphate esters are commonly used as a synthetic base stock for various functional fluids. Due to their low flammability and low density, phosphate esters are particularly useful as a base stock for aviation hydraulic fluids. Presently, a majority of the commercial aircraft in operation around the world employ hydraulic fluids based on phosphate esters, and worldwide production of these fluids currently exceeds one million gallons annually.
These commercial aircraft generate a significant quantity of used aviation hydraulic fluid, however, and disposal of this used fluid is problematic. Most used aviation hydraulic fluid is currently either incinerated or buried in landfills. However, incineration is difficult, and therefore costly, due to the limited flammability of such fluids; and landfill burial is environmentally undesirable due to the potential for leakage of such fluids into groundwater. Accordingly, it would be particularly desirable to reduce the volume of used fluid that must be disposed of by recovering at least one of the major components of the used fluid for subsequent reformulation or other uses.
In this regard, used phosphate ester-based aviation hydraulic fluids typically contain a major amount of a phosphate ester base fluid and a minor amount of one or more functional fluid additives. The base fluid generally contains a trialkyl phosphate ester, such as tributyl phosphate, and lesser amounts of one or more aryl-containing phosphate esters. Therefore, to significantly reduce the amount of used fluid that must be disposed of, it would be particularly advantageous to recover a phosphate ester, such as a trialkyl phosphate ester, from such used fluids.
Liquid trialkyl phosphate esters, such as those used in aviation hydraulic fluids, can typically be purified by distillation. However, distilling used aviation hydraulic fluid to recover a trialkyl phosphate ester has heretofore been complicated by at least two factors.
First, as noted above, used aviation hydraulic fluids typically contain a trialkyl phosphate ester, such as tributyl phosphate, and one or more aryl-containing phosphate esters. The boiling points of the particular phosphate esters used in these fluids are often quite similar, and therefore, their separation by distillation is exceedingly difficult. For example, tributyl phosphate and dibutyl phenyl phosphate, two phosphate ester species commonly present in aviation hydraulic fluid, have boiling points of 183.degree. C. and 200.degree. C., respectively, at 22 mm Hg. Consequently, distilling used aviation hydraulic fluid to separate a trialkyl phosphate ester from the aryl-containing phosphate esters present in the fluid requires costly high efficiency distillation equipment and exacting procedures.
As a result, such separations are currently uneconomical when compared to disposal.
A second factor that complicates the recovery of a trialkyl phosphate ester from used aviation hydraulic fluid by distillation is the fact that some of the additives present in such fluids are sufficiently volatile to co-distill with the trialkyl phosphate. For example, epoxide-containing hydrolysis inhibitors, which are frequently present in phosphate ester-based hydraulic fluids to prevent or reduce hydrolysis of the phosphate esters, often co-distill with the trialkyl phosphate ester thereby contaminating the recovered fluid.
In view of the above, a need exists for an effective process which allows a trialkyl phosphate ester to be recovered from used aviation hydraulic fluid such that the reclaimed trialkyl phosphate ester composition is essentially free of the aryl-containing phosphate esters and epoxide-containing hydrolysis inhibitors initially present in the fluid. The present invention provides such a process, in part, by transesterifing the aryl-containing phosphate esters present in the fluid to form a trialkyl phosphate ester; and additionally by chemically modifying the epoxide-containing hydrolysis inhibitors present in the fluid to facilitate the subsequent separation of the trialkyl phosphate ester from such additives.
The transesterification of phosphate esters is known in the art. For example, W. H. C. Rueggeberg et al. in The Journal of the American Chemical Society, 70, 1802 (1948) describe the reaction of ethyl phosphate with n-butanol in the presence of sodium butylate to form diethyl n-butyl phosphate and ethyl di-n-butyl phosphate.
Similarly, H. D. Orloff et al. in The Journal of the American Chemical Society, 80, 727(1958) describe, among other reactions, the transesterification of dimethyl phenyl phosphate with sodium methoxide to form trimethyl phosphate and sodium phenoxide.
Additionally, British Patent Application No. 1,027,059, published Apr. 20, 1966, indicates on page 1, lines 23-26 that it is known that triarylphosphates can be transesterified with alcohols or polyols by heating in the presence of alkali metal hydroxides or alkali metal alcoholates.
Other methods of recycling used phosphate ester-based hydraulic fluids are also known in the art. German Patent Application No. 2,704,469, published Aug. 10, 1978, for example, discloses a process for the reclamation of used phosphate-ester hydraulic fluid contaminated with decomposition products, such as phosphoric acid and phenolates. The process includes mixing the fluid with water and separating the aqueous phase until the neutralization number of the fluid is less than 0.8; mixing the fluid with aqueous alkali and bleaching earth at an elevated temperature; and then separating the sludge phase, suspended fines and residual water from the hydraulic fluid.